The present invention relates to an improved process for the preparation of ketimines. More specifically, this invention relates to processes for the production of ketimines having a higher level of purity. This invention also relates to the production of a ketimine-curing agent prepared by reacting a polyalkylene polyamine with methyl isobutyl ketone. The ketimines of the present invention having higher levels of purity are useful as curing agents and cross-linking agents for resins such as epoxy resins. This invention also relates to improvements in the production of ketimines by using specific weight ratios of reactants. The present invention also relates to the production of higher purity ketimine curing agents by careful control of reaction temperature. The invention is further directed to a process for the production of aliphatic ketimines having an unreacted polyalkylenepolyamine content of less than 1% by weight.
The invention relates to curing agents and cross-linking agents comprising a mono- or difunctional, one or more secondary amino groups-containing ketimines. Such curing agents are applied in bicomponent compositions curable under the influence of moisture and used for coating, impregnating, sealing and bonding purposes, on the basis of resins containing functional groups such as anhydride, epoxy, isocyanate, acetoacetate and xcex1,xcex2-ethylenically unsaturated carbonyl. The hydrolysis under the influence of moisture is attended with the release of a ketone, which can evaporate from the composition, and the formation of a primary amino group, which may enter into a cross-linking or curing reaction with the functional resins. Another application is to apply the ketimine curing agent under dried conditions and used for coatings or bonding purposes. The secondary amine functional groups of the Ketimine may enter into a cross-linking or curing reaction with functional resins, such as epoxy resins.
Ketimines derived from a variety of ketones and polyamines are described in U.S. Pat. No. 3,322,797. The ""797 patent describes the process for making the ketimine by reacting the polyamine with the ketone under refluxing conditions. The ""797 patent does not describe the importance of temperature and weight ratio of reactants in achieving a product having less than 1% free amine or unreacted polyamine content and less than 6% of other impurities including the tri-substituted species (trimers).
It is also known that in some cases the use of these low molecular weight ketimines gives rise to problems as far as toxicity, carbonization tendency and poor performance of the cured products. In those cases it is preferred that the molecular weight of the mono- or difunctional ketimine be increased by the addition to the secondary amino group(s) of one or more compounds reactive with the secondary amino group. Such xe2x80x9coligomericxe2x80x9d ketimines having a medium molecular weight, usually in the range of 300 to 3000, are described in, int. al., U.S. Pat. Nos. 3,975,251, 4,251,597, 4,503,174 and 4,504,630, the descriptions of which are herein incorporated by reference.
It is also known that the colour or at least the colour stability of the well-known curing agents is generally not quite satisfactory. Often the product is found to discolour during its preparation, while in other cases the products appear to discolour during storage. This is objectionable if they are to be used in compositions for which the colour is of essential importance, as with colourless coatings in, e.g., a 2-layer metallics system. The present invention provides ketimine curing agents and cross-linking agents, which are of higher purity and moreover display excellent colour and shelf stability.
Ketimines have been found to be particularly useful latent curing agents especially for polyepoxides where extending the potlife is desirable. The ketimines are relatively inert when mixed with polyepoxides, but when the mixture is exposed to a moist atmosphere they become highly reactive toward the polyepoxide and the mixture sets up to form a hard cured product. The purity of the ketimine plays a role in the pot life of the resulting compositions. Ketimines are typically formed by reacting a polyamine with a ketone. Limiting the amount of the polyamine in the final product is highly desirable since high levels of polyamines in the ketimines are detrimental to the pot-life when mixed with polyepoxides. The prior art is silent regarding ketimines wherein the content of polyamine is less than 1% by weight and the content of other impurities including trimers is less than 7.5% by weight and preferably less than 6% by weight. Therefore, there remains a long felt need for a process for making ketimines from the corresponding polyamines and ketones wherein the resulting ketimines have a polyamine content of less that 1% by weight and the content of other impurities including trimers is less than 6% by weight.